EP0461956A1 - Optical information storage on superimposed layers - Google Patents

Abstract

The invention relates to an optical information storage system.

The information is stored in the volume of a transparent layer (12) instead of being stored on the surface. Different sets of information are stored in different strata of the layer. The writing and reading means (40, 42) are designed to focus a light beam in the plane of a given stratum without focusing it on the others. Photodetectors (PD0, PD1, PD2) make it possible to selectively detect the beam disturbances generated by each of the respective individual strata.

The material of the transparent layer is preferably a material with photoinduced index variation.

Description

The invention relates to optical data storage.

One of the aims of the invention is to propose a data storage system having an even higher information storage capacity than in the systems currently existing.

One of the systems that allow very high storage capacity is the digital optical disc. Today, for example, 30 or 45 minutes of video images can be stored in a digital optical disc 30 centimeters in diameter. But we could only store a much shorter period if the images stored were high-definition television images. However, the principle of storage on optical disc would be very interesting for high definition television images as it is for conventional sound and image systems. We would then benefit from the advantages inherent in the system: contactless reading therefore wear-free, rapid random access by simple search for a track among a set of tracks. However, this requires a higher storage capacity than those that currently exist.

An improvement in the storage capacities is of course possible by using reading lasers of shorter wavelengths (for example 0.4 micrometer) than those which are commercially used (approximately 0.7 micrometer). The spatial resolution limit is in fact higher the shorter the wavelength.

The invention provides a means for further increasing the storage capacity of information in optical form.

According to the invention, it is proposed to store individual information in optical form in the volume of a layer of transparent material, different series of information being stored in different layers of the layer, reading information of a series being carried out by optical means capable of focusing a light beam in the plane of a determined stratum, chosen at will, without focusing it on the other strata.

In other words, the invention starts from the idea that the light beams which are used for reading optical discs are very fine and are focused by wide aperture objectives which have a very shallow depth of field. This small depth of field is conventionally taken advantage of by focusing the beam behind a transparent layer so that the impurities (dirt, scratches, etc.) on the surface of the discs are not "seen" by the beam which is not focused in their plane. According to the invention, the fact that the depth of field is very small is used to superimpose several layers of information, by providing means for moving the focal plane of the reading or writing optics so as to allow focusing. on a stratum chosen from the superimposed strata.

It should be noted here that the storage of images in the volume of a transparent layer already exists, but in the form of holograms. As we know, holograms are recordings of optical networks in transparent layers; they make it possible to reconstruct the visible image of an object when they are illuminated by a light coherent at the wavelength which served for the formation of the networks. But holograms are not means of storing individual information distinct from each other. They use the whole volume of a transparent layer to store a global image which is not dissociated into individual pieces of information and which can only be reproduced globally. The invention is, on the contrary, only interested in the storage of information point by point, that is to say a storage in which each element of information is autonomous and has its own meaning which can be dissociated from other elements of information and can therefore be read separately from the others.

Preferably, for the storage of information in strata according to the invention, a transparent layer made of a material with variation in photoinduced refractive index, of the same kind as those used for holograms, will be used.

These materials are transparent and their optical index is likely to change locally under the effect of a local concentration of light energy. The change in index is small but sufficient to be detected and allow reading.

The materials preferably used will be photopolymerizable materials. One of the advantages of these materials is that they can record information in optical form without going through a chemical development phase.

Preferably, one of the information strata constitutes a reference track with respect to which the other strata will be referenced. This layer can be produced differently from the others, and optionally by conventional pressing from a matrix obtained initially by photoengraving.

The storage layer will preferably be formed on a disc which, in read mode, will be rotated below a laser beam focusing optic, this optic being capable of forming focusing spots at several different depths below of the surface of the disc. Of course, the focusing optics can also be moved radially relative to the disc (in a conventional manner) to ensure reading over the entire radial extent of the disc.

To ensure the serial production of discs, one can make a master disc comprising the different strata of information written by a laser, make a hologram of the three-dimensional image formed by the transparent layer containing the information in strata, then reconstruct an image. three-dimensional by means of a laser beam illuminating the hologram and projecting the reconstituted image on a transparent layer with variation of photoinduced index, so as to create in this layer photoinduced index variations reproducing the pattern of information contained in the corresponding layer of the master disc.

In summary, the invention presented here has several aspects: a new information storage system, but of course also a medium for storing information in strata, a corresponding reading system which allows the separation of the information contained in superimposed strata , and also a method for producing optical storage media in series.

• la figure 1 représente globalement un disque optique selon l'invention;
• la figure 2 représente le principe de l'écriture d'informations par strates dans une couche épaisse;
• la figure 3 représente schématiquement une coupe de disque selon l'invention avec les informations enregistrées par strates;
• la figure 4 représente une vue en perspective correspondant à la figure 3;
• la figure 5 représente le système de lecture des informations enregistrées;
• la figure 6 explique schématiquement le principe d'une lecture différentielle
• la figure 7 explique une méthode de lecture par réflexion préalable.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:

• FIG. 1 shows an overall optical disc according to the invention;
• FIG. 2 represents the principle of writing information by strata in a thick layer;
• FIG. 3 schematically represents a section of a disc according to the invention with the information recorded by strata;
• Figure 4 shows a perspective view corresponding to Figure 3;
• FIG. 5 represents the system for reading the recorded information;
• Figure 6 schematically explains the principle of differential reading
• FIG. 7 explains a method of reading by prior reflection.

The optical recording system according to the invention uses, for the initial recording of the information, a thick layer of transparent material in which the individual information will be arranged in superimposed layers.

By thick layer is meant a layer much thicker than the beam focusing spot. laser which will be used to register the information. As we can estimate that the focusing spot has dimensions of the order of magnitude of the wavelength of the beam (or sometimes even a little less), the thickness of the layer will be at least several tens of times The wavelength. For visible light wavelengths, the focusing spot (which we seek to reduce as much as possible) can extend over lateral dimensions of the order of a micrometer; the thickness can then be of the order of one or several hundred micrometers.

This layer is preferably formed on a flat support disc which can be rotated about an axis.

FIG. 1 schematically represents the disk 10 with the transparent layer 12 which covers it. The support is for example made of glass. It is optionally possible to provide, between the support 10 and the information storage layer 12, a reflective or partially reflective layer, not shown.

The material of layer 12 is, in this embodiment, a photopolymerizable material which has a variation in photoinduced index. A laser beam of sufficient energy, focused on a small area inside the layer can locally modify, in this area, the refractive index of the layer. The index variation can go up to 10%, or even more. The modification is strongly nonlinear, that is to say that there is an energy threshold below which there is practically no variation and beyond which there is a net variation.

We are going to photoinduce very localized variations of index at different depths below the surface of the disc; the different depths define strata of recorded information. The information is in a certain way binary, each local modification of index representing an information bit. This does not exclude, of course, that we consider a storage of information which is in fact analogical: for example, the useful information would be the length of an area with a local index change, or the distance between two successive local index changes.

As will be explained below, the transparent layer can be covered with a layer 14 of etched resin comprising information and constituting, for example, a reference track for reading information from the interior layers.

For the recording of information, an optical recording stylus is used comprising either an optic movable vertically relative to the surface of the disc, or fixed means for producing several focusing spots at different depths.

FIG. 2 represents an exemplary embodiment of the recording means, with a single laser 20, of the means for directing a part of the laser beam on several partially transparent mirrors M1, M2, M3, situated at different heights above the surface of the disc, and a focusing objective 22 producing very narrow focusing spots (dimensions of the order of the wavelength or half the wavelength of the beam) at depths directly linked to the respective heights of the mirrors above of the disc.

For example, the beam returned by the mirror M1 produces a spot S1 in a plane P1, the beam returned by the mirror M2 produces a spot S2 in a plane P3, etc.

The heights of the mirrors are calculated so that the planes P1, P2, etc. are separated by very precise distances, of the order of several tens of micrometers for example.

The transparent layer 12 rotates below the laser beam when the disc rotates.

The optics can also be moved radially relative to the axis of the disc. In this way, the entire useful surface of the disc can be scanned by the recording laser beam, and this for each of the information layers.

Light modulators (Pockels cells or other) are interposed on the paths of the laser beams corresponding to each of the focusing spots. These modulators MD1, MD2, etc. therefore correspond to the various layers of information to be recorded in the volume of layer 12. They are controlled to allow the beam to pass or interrupt according to the information to be entered; an index variation is produced or is not produced. Synchronization is provided between the modulation command and the relative movement of the disc and of the focusing spots, so that the information is written with a well-defined spatial distribution.

The spacing of the recorded points is related to the resolution of the disc, that is to say in practice to the lateral dimension of the focusing spots that can be produced with the lens 22. Two points must be separated by a distance of the order of this lateral dimension of the spot, for example of the order of the wavelength or the half wavelength of the laser beam.

The spacing in depth is dictated by the possibility of reading the information afterwards. It can be said that the spacing between strata must be several times or several tens of times the vertical dimension of the focusing spot so that a given stratum does not create information interference with the neighboring strata. In this way, the light energy, sufficiently concentrated in a point of a given stratum to produce a variation of index, is defocused in the other strata and does not produce variations of index.

The recording system described with reference to FIG. 2 is given only by way of example: it can very well be provided that there is only one mirror and one optical modulator; the entire focusing optic is then movable relative to the surface of the disc to focus the beam at a variable depth below the surface of the disc. And the strata are recorded successively rather than simultaneously.

We can foresee that the disc includes an upper layer of resin in which we will burn, by photolithography, an information reading track which will serve as a reference for the inner layers: the information of each layer will be located on tracks located below the reference tracks of the photograved resin layer.

After recording and developing the photosensitive resin, corresponding for example to an information plane P0, the disc has a structure such as that of FIG. 3. On the surface there is a photo-etched track (similar to that of an optical disc classic) with information constituted by holes located in the resin 14; and in depth, there are tracks distributed in the different focus planes Pi and aligned vertically with the surface track. Along these interior volume tracks are photoinduced pieces of information, characterized by an index N + dn locally different from the index N of the layer.

FIG. 4 represents a schematic perspective view corresponding to the structure of FIG. 3.

It will be noted here that certain materials can undergo a heat treatment which increases the relative difference between the index N + dn locally modified and the index N of the rest of the layer.

A disc containing a large amount of information was thus produced. It can be used as is to read and use, but it can also be used as a master disc for serial reproduction of discs containing the same information.

To read the master disc thus produced, or to read the copy discs obtained by duplicating the master disc, the procedure is as follows:

The disc is inserted into a playback system. this system includes means for focusing a laser beam on the different layers of information provided in volume in the transparent layer covering the disc. We can have only one beam and vertical displacement means; or several beams each establishing a respective focusing spot. The laser beam of course has an energy low enough not to create index changes in the layer containing the information.

In Figure 5, there is shown a reading system with several beams and separate optical means for producing the different focusing spots. The beam is produced by a laser 40 and is divided by a separator 42 into several beams which can be guided for example by optical fibers 440, 441, etc. towards a respective focusing optics 450, 451, 452; this optic focuses the different beams on respective partially transparent mirrors 460, 461, 462, from which they are returned to a lens 48 which focuses them in the transparent layer 12 of the disc 10, each at a respective depth within the layer (the depth corresponding each time to a respective information layer).

For each of the beams, the presence of a local index variation in the layer produces a partial reflection of the beam towards the rear. The portion of the beam reflected at the location of the focusing spot is returned to the objective 48 and is focused on the mirror 460, 461, 462, etc. corresponding to the beam considered. From there, the reflected beam is returned to a respective auxiliary semi-transparent mirror 500, 501, 502, etc. interposed between objective 450, 451, 452, etc. and the mirror 460, 461, 462, etc.

Finally, the reflected beam, returned by the respective auxiliary mirror, is directed onto a photodetector PD0, PD1, PD2, etc., each photodetector corresponding to a respective information stratum and supplying a signal corresponding to the information read in this stratum.

The strong focusing of the light beam, producing focusing spots of dimensions much smaller than the distance separating two layers of information, makes it possible to collect the information contained in a stratum, without interference with neighboring strata; the neighboring strata are seen in a "fuzzy" manner by the beam, so that the information which they contain does not significantly disturb the reading of the considered stratum.

The read head control will be obtained by a conventional method of radial and vertical control (used in all conventional optical discs), preferably using the surface track P0 etched in the resin 14 as a reference.

The system described, for the optical transmission of an illumination beam and the return of the reflected beam is given only by way of example, many other possibilities that can be envisaged, in conventional optics or with optical fibers.

The reading source can be monochrome or broad spectrum, or even made up of several sources of different wavelengths, each wavelength being for example assigned to a respective information layer; in the latter case, it will be possible to filter the reflected beams so as to correspond to each photodetector only a clearly determined wavelength.

When the laser beam encounters a local index variation of around 10% at the location of the focusing spot, it can be estimated that about 1 thousandth of the beam energy is reflected backwards. It is this fraction which is collected by the photodetectors to give an output signal representing the information read.

Since this value is relatively low, it is preferable to use differential detection methods using the phase shift of light introduced by the reflection of the beam on index variation planes locally having excess thicknesses. This method is explained below, with reference to Figure 6.

An information layer inside the transparent layer 12 is defined by a continuous internal layer in which the refractive index has been continuously modified at the time of recording. The index is N + dn in this layer. In addition, the layer of index N + dn has local excess thicknesses whose presence or absence represents the information recorded.

When the reading laser beam is completely reflected on the layer of index N + dn or on an additional thickness, light is reflected backwards. But when the beam is reflected half on the layer and half on a local allowance, the portion of beam reflected by the allowance is out of phase with that which is reflected by the rest of the layer. This phase shift is linked to the ratio between the height e of the allowance and the wavelength; it is also proportional to the variation of index dn. If the height e of the extra thicknesses is correctly chosen as a function of the wavelength and of the variation of the index dn, a phase shift of 180 ° is obtained. There is then a brief instant during which the beam, in relative rotation with respect to the extra thicknesses, passes over an edge of an extra thickness, half of the beam being reflected with a phase shift of 180 ° with respect to the other half. There is a momentary "extinction" of the reflected beam, and this extinction can be detected.

Another reading method can be used; it is illustrated in figure 7.

It consists in using a reading by reflection on the rear face of the transparent layer, in such a way that the information is in fact analyzed in transmission: the focusing spot is produced in the information plane but after reflection on the rear face. In the absence of local index modification, the entire beam crosses the layer and is brought back to the photodetectors; in the presence of a local index change, the beam is diffracted and a lesser amount of energy is collected by the photodetectors. The photograved resin track can also be located on this rear face as shown in FIG. 7.

Since it will often be desirable to mass produce discs with identical information, there are two ways to do this.

The first way consists in producing the discs all in the manner which has been described with reference to FIG. 2, with an inscription laser which writes point by point in each of the layers of the transparent layer.

The second way, which is preferable for large series, consists in producing a master disc from a laser inscription as in FIG. 2, then in producing by duplication in series copy discs from the master disc. The duplication process is therefore global and not point by point.

For serial duplication by a global process, the invention proceeds as follows: the master disc is used to produce a volume hologram in a photosensitive material (which can moreover be fixed by chemical development so as to later become insensitive to light). The hologram produced, lit by a plane wave of coherent light with high energy, makes it possible to reconstruct a two-dimensional image of the master disc with the information it contains; this image is projected onto a copy disc covered with a transparent layer with photoinduced index variation. This high light energy projection causes the desired local index changes in the transparent layer of the copy disc, thus reproducing globally a faithful copy of the master disc with all the information it contains.

If the master disc has a surface track produced by etching a photosensitive resin and not by variation of photoinduced index, copy discs will preferably be produced comprising a corresponding surface track obtained by pressing. The pressing technique is the technique classic for today's digital optical discs. Pressing can be done before or after exposure through the hologram.

Claims (7)

Optical information storage system, characterized in that: the information is stored in the volume of a layer of transparent material (12), different series of information being stored in different strata of the layer, the information of a series being accessible for reading from optical means (40, 42, etc.) capable of focusing a light beam in the plane of any chosen stratum without focusing it on the other strata. Optical information storage system according to claim 1, characterized in that the layer of transparent material (12) is formed on a flat disc capable of being rotated about an axis perpendicular to its plane. System according to claim 2, characterized in that the transparent material is a substance with photoinduced index variation. Method for writing information in a system according to one of Claims 1 to 3, characterized in that it comprises the steps consisting in: - make a master disc with a layer containing the information, - make a hologram of the master disk layer, - reconstruct a three-dimensional image of the layer of the master disk from the hologram and a coherent beam of light, - Project the reconstituted image on a disc covered with a transparent layer with variation of photoinduced index and thus create in the transparent layer a replica image of the layer containing the information. Information medium for implementing the method according to one of claims 1 to 4, characterized in that it consists of a flat disc covered with a transparent layer comprising individual information stored in optical form and distributed according to several layers inside the volume of the layer, each layer corresponding to a respective series of information. Support according to claim 5, characterized in that the disc has one face with an engraved track serving as a reference for accessing the information contained in the volume of the transparent layer. System for reading optical information stored in a layer of transparent material (12), characterized in that it comprises optical means (40,42) capable of focusing a light beam in the plane of any chosen stratum of the layer, among several possible strata, and means for detecting disturbances of the beam caused solely by the information stored in the chosen stratum.

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